Geomorphology 81 (2006) 114 – 127
www.elsevier.com/locate/geomorph
Geologic versus wildfire controls on hillslope processes
and debris flow initiation in the Green River canyons
of Dinosaur National Monument
Isaac J. Larsen a,⁎, Joel L. Pederson a , John C. Schmidt b
b
a
Department of Geology, Utah State University, Logan, UT 84322-4505, USA
Department of Aquatic, Watershed, and Earth Resources, Utah State University, Logan, UT 84322-5210, USA
Received 3 October 2005; received in revised form 5 April 2006; accepted 9 April 2006
Available online 13 June 2006
Abstract
As in many areas of high relief, debris flows are an important process linkage between hillslopes and the Green River in the
canyons of the eastern Uinta Mountains, yet the physical conditions that lead to debris flow initiation are unknown. A recent
episode of enhanced debris-flow and wildfire activity provided an opportunity to examine the geomorphic impact of fire and the
processes by which weathered bedrock is transported to the Green River. Field investigations and analysis of elevation and
precipitation data were undertaken in 15 catchments with recent debris flows to determine how surficial geology, wildfire,
topography, bedrock strength, and meteorology influence hillslope processes. The recent debris flows were triggered by intense
summer rainstorms. The dominant debris flow initiation mechanism, the firehose effect, occurred when overland flow generated on
bedrock hillslopes cascaded down steep cliffs onto colluvium, causing failure. Sixty percent of the debris flows occurred in
unburned catchments. However, 15% of the burned catchments in the study area produced debris flows over the study period,
whereas only 7% of the unburned catchments did. Thus, fire was not the primary driver of debris flows, but fire-related events did
contribute to the increased debris flow activity. The geomorphic impact of wildfire in the eastern Uinta Mountains is not as great as
in transport-limited settings with regolith-mantled hillslopes. The strong rocks and dry climate of the study area cause hillslopes to
be very steep and weathering-limited, with high runoff ratios and a dearth of regolith. As a result, there is little vegetation, and thus
we hypothesize that burning does little to change hillslope processes. The suite of hillslope processes in the eastern Uintas are like
those documented in the similarly dry Grand Canyon, but differ from other locations in the western U.S. where wildfire is a
primary control on debris flow processes.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Debris flows; Wildfire; Bedrock; Eastern Uinta Mountains; Green River
1. Introduction
⁎ Corresponding author. Current address: Department of Forest,
Rangeland, and Watershed Stewardship, Colorado State University,
Fort Collins, CO 80523-1401, USA.
E-mail address: [email protected] (I.J. Larsen).
0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2006.04.002
Debris flows can be significant geologic hazards and
an important geomorphic process in high-relief landscapes (Costa and Weiczorek, 1987). Debris flows form
an important link between hillslopes and channel
networks, because they transport large volumes of
I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
sediment from hillslopes to adjacent valleys (Webb et
al., 1988; Benda, 1990; Grant and Swanson, 1995;
Webb et al., 2000; Benda et al., 2003; Eaton et al.,
2003). Debris flows also play a key role in shaping the
morphology of the mainstem valley into which they
flow, as is the case with the Green River in the canyons
of the eastern Uinta Mountains in Dinosaur National
Monument. There, coarse-grained debris flow deposits
create fan–eddy complexes, which are the fundamental
geomorphic unit of debris–fan affected canyons
(Schmidt and Rubin, 1995).
Fan spacing and shape determine the locations of
expansion gravel bars, eddy bars, and channel-margin
deposits. Close spacing of debris fans causes steeper
canyon-scale gradient (Grams and Schmidt, 1999;
Larsen et al., 2004; Magirl et al., 2005). Since 1997,
debris flows in 15 tributaries aggraded fans and rapids in
the Green River corridor in Dinosaur National Monument (Martin, 2000; Larsen, 2003). Several of these
debris flows deposited coarse sediment within the
mainstem channel, altering the morphology of the
Green River (Larsen et al., 2004).
Despite the importance of debris flows to channel
morphology in the Colorado Plateau, the processes that
initiate debris flows and thereby link hillslopes in
tributary catchments with the mainstem valley of the
Green River are poorly understood. No debris-flows
were documented between 1993 and 1997 (Grams and
Schmidt, 1999). The recent debris flow activity in
Dinosaur National Monument might be linked to
wildfires, because wildfires burned a total of 34 km2
in several tributary catchments of the Green River in
1996 and 2001.
Fires are either viewed as a significant contributor to
debris-flow generation or as an insignificant factor in the
Intermountain West. For example, fires are a significant
factor in debris-flow initiation in the Yellowstone region
and west-central Colorado because fires remove protective organic litter from soil-mantled hillslopes and
increase overland flow production during rainfall events
(Meyer and Wells, 1997; Cannon et al., 2001b). In
contrast, fires do not contribute to debris-flow activity in
the Grand Canyon region. Rather, high intensity rainfall
on steep, rocky hillslopes generates infiltration-excess
overland flow that leads to colluvium failure and debris
flows (Griffiths et al., 2004).
The recent debris-flow and wildfire activity in the
eastern Uinta Mountains, located at the interface
between the Colorado Plateau and the Rocky Mountains, provides an opportunity to examine the physical
factors that lead to debris-flow generation and to
assess the geomorphic significance of wildfires. Here,
115
we describe process linkages among bedrock strength,
topography, colluvium storage, and debris-flow initiation and we describe the influence of wildfire on
hillslope processes in this steep, weathering-limited
environment.
2. Debris flow initiation processes
Debris flows are a type of granular mass flow whose
motion is governed by changes in Coulomb friction and
internal pore-fluid pressure (Iverson, 1997). Debris-flow
initiation can occur from failure of bedrock, colluvium,
or alluvium (Costa, 1984). The specific physical
conditions that trigger debris flows vary greatly, because
debris flows occur in diverse physiographic and climatic
environments.
Rock strength influences debris flow activity because
it imparts a control on both the susceptibility of bedrock
slope failure and the local production rate of colluvium
(Wohl and Pearthree, 1991; Schmidt and Montgomery,
1996; Coe et al., 1997; Bovis and Jakob, 1999).
Landscape steepness and the extent to which colluvium
covers bedrock on hillslopes influence the regional
susceptibility of debris flows. However, measurements
that relate bedrock strength and colluvium cover to
debris-flow generation are relatively few.
Debris flows most commonly mobilize from landslides, which occur when rainfall elevates pore pressure
and adds weight to the hillslope, causing failure (Varnes,
1978; Caine, 1980; Wieczorek, 1987). However,
streamflow falling onto colluvium can also trigger
debris flows through a process called “the firehose
effect” (Johnson and Rodine, 1984). The firehose effect
commonly occurs in ephemeral channels in cliff-andbench topography where colluvium accumulates below
cliff bands. High intensity rainfall leads to floods
(Fryxell and Horberg, 1943; Berti et al., 1999; Griffiths
et al., 2004) that pour over cliffs or bedrock steps
directly onto colluvium, which fails and mixes with the
water to form a debris flow (Griffiths et al., 2004).
Although rainfall is an important precursor to debris
flow initiation whether mobilization occurs from
saturated landslides or the firehose effect, the intensity
and duration of the rainfall events that trigger debris
flows are difficult to determine in dry environments
(Griffiths et al., 1997; Ben David-Novak et al., 2004).
Fires decrease infiltration, increase overland flow
rates, and lower the threshold of entrainment of
sediment from hillslopes (e.g., Swanson, 1981; Wondzell and King, 2003; Moody et al., 2005). These
conditions increase watershed susceptibility to debris
flows, especially via the process of progressive sediment
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bulking (Parrett, 1987; Wohl and Pearthree, 1991;
Meyer and Wells, 1997; Cannon and Reneau, 2000;
Cannon et al., 2001a,b; Meyer et al., 2001). In this case,
increased overland flow causes extensive rill and gully
erosion on burned hillslopes. The overland flow entrains
and transports large concentrations of sediment into the
drainage network and scours channels. A debris flow
forms when sufficient eroded material is entrained
relative to the volume of water (Meyer and Wells, 1997;
Cannon et al., 2001b). Our goal is to assess the relative
importance of these geologic, hydrologic, and wildfire
controls on debris flow initiation in the steep, dry
canyons of the eastern Uinta Mountains.
3. Study site
The Green River flows across the eastern Uinta
Mountains, forming three spectacular canyons: Canyon
of Lodore (hereafter referred to as Lodore Canyon),
Whirlpool Canyon, and Split Mountain Canyon (Fig. 1).
In the high-relief topography adjacent to the river,
tributary headwaters occur near the elevation of the
canyon rim and drain to the Green River in steep
ephemeral channels that fall hundreds of meters to the
canyon floor (Fig. 2).
The Green River flows southward, roughly perpendicular to the axis of the east–west trending anticline of
the Uinta Mountains (Hansen, 1986). The anticlinal
structure, along with local faulting, causes the elevation
of bedrock units to vary. The Neoproterozoic Uinta
Mountain Group orthoquartzite forms the canyon walls
in the northern part of Lodore Canyon. The base of the
lowest exposed Paleozoic sedimentary bedrock occurs
at the top of the canyon walls in the central part of
Lodore Canyon. Paleozoic bedrock (including the cliffforming Madison limestone, Upper Morgan Formation,
and Weber sandstone), as well as slope-forming shales
of the Lower Morgan Formation, occur at progressively
lower elevation on the canyon walls in the southern part
of Lodore Canyon. Near the end of Lodore Canyon, the
Fig. 1. Map of Dinosaur National Monument and nearby meteorological stations.
I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
117
Fig. 2. Simplified surficial geology of study catchments in Lodore and Whirlpool Canyons based on mapping completed during this study. Hatched
lines show the areas that burned in 1996 and 2001. The letters adjacent to each catchment correspond to data in Table 1.
canyon walls are locally composed of vertically bedded
Paleozoic bedrock resulting from deformation associated with the Laramide Mitten Park Fault. The southern
end of Lodore Canyon occurs where the Yampa River
joins the Green River in Echo Park. Further downstream
is Whirlpool Canyon (Fig. 2), where the same Paleozoic
rocks occur in tributary catchments. Bedrock structure
controls drainage patterns in the area, and nearly all
major tributaries and many of the smaller drainages are
subsequent to these structures (Grams and Schmidt,
1999).
The semiarid climate of the eastern Uinta Mountains
supports a pinyon-juniper (Pinus edulis–Juniperus
osteosperma) woodland. Organic litter accumulates
directly beneath tree canopies, but intercanopy spaces
are characterized by a sparse undergrowth of grasses.
Mean annual precipitation ranges from 21 to 30 cm,
35% of which occurs between June and September.
Summer rainfall is greatest on the southern and eastern
flanks of the mountain range and is related to relatively
moist monsoonal air masses that move northward and
orographically rise over the Uinta Mountains (Harper et
al., 1981).
4. Methods
Field investigations took place in the 15 catchments
that experienced recent debris-flow activity (Fig. 2).
Surficial geology and bedrock were mapped at 1:12,000
scale in each study catchment and digitized into a
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I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
geographic information system (GIS). Debris-flow
initiation sites were identified by field inspection or by
locating the sites from distant vantage points where
steep topography made foot access impossible. In both
cases, each debris-flow initiation site was located on
1:12,000 scale aerial photographs and digitized into a
GIS. The initiation sites were identified between one
month and five years after the debris flows occurred.
Based on field evidence, the debris flow initiation
process was assigned to one of three categories: i) the
firehose effect, ii) shallow landsliding, or iii) progressive sediment bulking. Field evidence for firehose-effect
debris-flow generation included evidence of streamflow
upstream from the cliff band or bedrock step and
evidence for a debris flow that initiated in colluvium
immediately below. Initiation sites were marked by
freshly scoured, or failed, colluvium located within or
near an ephemeral channel, and levees and mudlines on
scoured gully walls extended downstream from colluvium failure points. Upstream from the cliff bands,
evidence for streamflow included the presence of wellsorted deposits and lack of mudlines or significant scour.
Field evidence for shallow landslide initiation of debris
flows included tension cracks in colluvium, steep,
planar failure surfaces, and exposed, underlying bedrock. Evidence for progressive sediment bulking
included the presence of rills on lower hillslopes,
increasing channel scour in the downstream direction,
and the presence of debris-flow levees further downstream. At sites where progressive sediment bulking
triggered debris flows, the initiation site was mapped at
the point where levees first appeared. Field topographic
surveys of the debris fan at the outlet of each watershed
were the basis for estimating the volume of each recent
debris-flow deposit, similar to methods of Webb et al.
(1999). The volume estimates are considered minimum
values; some deposits were subsequently eroded by the
Green River and deposition sometimes occurred
upstream from the watershed outlet.
Burn severity in the areas burned by wildfire in 2001
was classified based on the degree of organic litter
combustion and alteration in the color of mineral soil,
following the criteria of Wells et al. (1979). Because
these criteria are difficult to apply several years
following wildfire, the degree of tree mortality was
used to categorize burn severity in the areas that burned
in 1996.
Soil–water repellency is often cited as a cause of
increased overland flow following fires (e.g., DeBano,
2000; Letey, 2001), and we evaluated its likelihood in
the study area. Soil–water repellency was measured
using the critical surface tension method to determine if
soils were likely to become water-repellent following
burning (Watson and Letey, 1970; Huffman et al.,
2001). The measurements were made within one month
of a summer 2002 wildfire that burned 19 km2 in
Dinosaur National Monument, most of which drains to
the Yampa River. Soil–water repellency could not be
evaluated for older fires, because the strength of water
repellency can decrease dramatically within one year of
burning (MacDonald and Huffman, 2004).
Slope, area, and elevation data collected from a 30-m
digital elevation model (DEM) were used to calculate
mean catchment slope, the slope of each initiation site,
the topographic contributing area upstream from each
initiation site, and catchment hypsometric integrals. We
used a chi-square test to determine if there was a
significant difference in the frequency of debris flows
from burned versus unburned catchments.
Rock-mass strength was evaluated for nonshale
bedrock units using semiquantitative methods similar
to those of Selby (1980). Rock elastic rebound for each
bedrock unit was measured at ≥ 50 points with a
Schmidt Hammer. Schmidt Hammer data were combined with observations and measurements of weathering and joint characteristics to categorize the strength of
each bedrock unit. Values of shale strength were
estimated from published values (Selby, 1980).
Twenty-five samples of shale, colluvium, and recent
debris-flow material were collected for clay mineral
analysis to determine if there was a link between the
spatial distribution of clay-bearing strata and recent
debris-flows (Larsen, 2003). Samples of oriented clays
were glycolated and X-rayed to identify clay minerals
(Moore and Reynolds, 1997). The X-ray patterns were
modeled to quantify the relative percentages of illite,
kaolinite, and smectite in each sample using the
NEWMOD software (Reynolds and Reynolds, 1995),
with an accuracy of ± 5% (Moore and Reynolds, 1997).
The proportion of sand, silt, and clay in each sample was
determined by the hydrometer method (Gee and Bauder,
1986).
Eyewitness accounts provided data concerning the
timing of some debris-flow events, but many of the
debris flows were first discovered several days after they
occurred. Thus, the precision of the estimated date of
debris flow occurrence is between 1 and 7 days. In the
instances where there is poor precision, the timing of
debris-flow activity was estimated from rainfall records
from nearby gages. Daily precipitation records from
three weather stations surrounding the field area
(NCDC, 2003) were used to determine daily rainfall
totals for all debris flow-events (Fig. 1). Hourly
precipitation data collected at two Remote Automated
I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
119
Weather Stations (RAWS) were used to calculate
maximum 1-h rainfall (e.g., 12:00–13:00) for each
debris flow-producing storm (WRCC, 2003). The 24and 1-h rainfall totals were compared with a precipitation atlas (Miller et al., 1973) to determine the
recurrence interval of each debris flow-producing
rainfall event. Summer, defined here as June–September, rainfall totals for 1993–2002 were compared with
historic mean rainfall to investigate whether debris flow
activity was related to seasonal rainfall.
Canyons, and several fresh scars on bedrock were
indicative of recent rockfall activity (Larsen, 2003).
Colluvium also occurs on bedrock ledges within the cliff
and bench topography of the canyons and, at a smaller
scale, within bedrock hollows located in the axes of
steep drainages. Shales typically form lower-angle
slopes that are the storage location of colluvial wedge
deposits that may be as much as 3 m thick.
5. Results
Eleven of the 15 debris flows that reached the Green
River between 1997 and 2002 occurred in Lodore
Canyon and four debris flows occurred in Whirlpool
Canyon (Fig. 2). Twelve of the 15 recent debris flows
initiated via the firehose effect. Two debris flows were
initiated from shallow landsliding within burned areas
and one was initiated from sediment bulking on an
unburned hillslope.
Debris flows that initiated via the firehose effect
began when overland flow generated in the catchment
headwaters cascaded down steep channels or over cliffs
onto colluvium stored in hollows and on bedrock
benches (Fig. 3). The colluvium failed, initiating debris
flows that continued down-channel (Fig. 3). Failure
5.1. Surficial geology
Detailed mapping in the catchments with recent
debris flows indicates that the proportion of bedrock (or
bedrock with thin, discontinuous colluvium) ranges
from 19% to nearly 100%, but is generally between 50%
and 80% (Fig. 2, Table 1). The hillslopes are thus
weathering-limited in terms of sediment supply, like
hillslopes in many dry environments (e.g., Yair and
Enzel, 1987; Pederson et al., 2001). Colluvium derived
from rockfall and other mass-wasting processes occurs
at the base of cliffs throughout Lodore and Whirlpool
5.2. Initiation of debris flows
Table 1
Characteristics of the catchments with recent debris flows
Date of
River
kilometera debris
flow
385.0-A
384.8-B
381.8-C
380.6-D
378.7-E
373.0-F
368.6-G
367.0-H
363.5-I
363.5-J
362.1-K
356.8-L
353.1-M
352.3-N
351.5-0
a
Catchment Contributing
areab
area above
(km2)
initiation
siteb (km2)
8/15/2001 0.25
8/15/2001 0.11
8/15/2001 0.74
8/15/2001 10.71
9/19/1997 0.88
9/19/1997 0.27
6/17/1998 2.48
9/19/1997 0.95
8/15/2001 0.09
8/15/2001 0.04
7/25/2002 0.03
7/25/2002 0.01
7/30/1999 1.51
7/30/1999 1.13
7/30/1999 0.98
0.04
0.01
0.14
10.2
0.26, 0.01, 0.16, 0.16f
0.02
0.07
0.09
0.01
0.01
0.003
0.01
0.04
0.10
0.07
Slope of
initiation
site
(m m− 1)
Year
burned
Area
Bedrock Mean
Hypsometric Deposit
volume
burned area
catchment integrald
(m3)
(%)
(%)
slopec
(m m− 1)
1.1
2.7
1.2
0.5
1.0, 1.0, 1.2, 1.2f
0.7
1.0
0.5
1.0
0.8
0.5
0.6
0.7
0.6
0.6
Unburned
2001
2001
2001
1996
Unburned
1996
1996
Unburned
Unburned
Unburned
Unburned
Unburned
Unburned
Unburned
–
55
92
99
74
–
55
63
–
–
–
–
–
–
–
92
79
90
84
77
79
19
48
69
86
94
100
81g
62g
56g
0.8
1.0
0.6
0.4
0.5
0.8
0.4
0.6
0.6
0.6
0.5
0.5
0.4
0.6
0.5
0.74
0.56
0.63
0.59
0.69
0.62
0.70
0.60
0.57
0.56
0.58
0.65
0.78
0.67
0.71
25
240
875e
770
37.5
30
7.5
2100
24
10
54
18
150
4300e
30
River kilometers measured from the confluence of the Green and Colorado Rivers, distances increase upstream, letters match locations in Fig. 2.
Calculated with D-infinity algorithm (Tarboton, 1997).
c
The average slope of all of the DEM grid cells within the catchment.
d
The hyposometric integral is the area beneath the curve in a plot of normalized catchment elevation versus normalized catchment area (Strahler,
1952).
e
Volume includes only deposits in the Green River; excludes large deposits on fan surface.
f
There are four separate initiation sites in the river kilometer 378.7 catchment.
g
One-third to one-half of which is Tertiary Brown's Park Formation.
b
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I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
accommodate sliding, rotated colluvial blocks, and
hummocky topography within the burned area and in
adjacent unburned portions of the catchments indicate
that landslide activity began prior to the recent fires
(Larsen, 2003). The recent debris flows were triggered
by failure of material located within the toes of these
much larger, pre-existing landslides.
A third initiation type occurred at river kilometer
356.8 (Fig. 2) where infiltration-excess overland flow
led to rill and gully erosion on an unburned hillslope,
resulting in debris flow generation via sediment bulking.
The hillslope at river kilometer 356.8 is adjacent to the
Mitten Park Fault and is composed of vertically dipping
Lower Morgan Formation shale, with a few limestone
beds that are < 0.5 m thick. Infiltration-excess overland
flow on bare, unburned shale slopes led to rill erosion in
the catchment headwaters and gully erosion of regolith
where overland flow was confined by resistant limestone beds in the downstream portion of the catchment.
Levees were first present ∼ 30 m downstream from the
drainage divide, indicating debris flow passage from
high in the watershed.
5.3. Topographic and bedrock characteristics
Fig. 3. The debris flow initiation site located at river kilometer 384.8 in
Lodore Canyon. The debris flow initiated when overland flow
generated on dominantly bedrock slopes at the canyon rim cascaded
down a cliff onto colluvium. The debris flow continued downslope
where it scoured colluvium from downslope storage areas and
deposited 240 m3 of material on a debris fan. Fifty-five percent of
the watershed burned in 2001. The area where colluvium failed is
circled.
volumes were not surveyed, but measured deposit
volumes were typically greater than the visual estimates
of colluvium failure volumes. This indicates that debris
flows increased in volume after failure by entraining
downstream colluvium.
Exceptions to the firehose effect mechanism were
observed in three study catchments. Debris flows
initiated from shallow landsliding in two catchments
in Lodore Canyon, at river kilometers 367.0 and 368.6
(Fig. 2). Shallow landslides completely removed 1–2 m
of colluvium from the hillslopes, exposing underlying
shale. Isolated blocks of intact colluvium on the failure
plane and tension cracks in upslope colluvium suggest
the landslides were translational in nature. The debris
flows in these two catchments likely initiated when
storms wetted the colluvium, causing blocks of
landslide material to slide off the shale slope and over
steep cliffs where they mobilized into debris flows. This
area burned at moderate severity in 1996. However, the
presence of springs, trees with trunks that bend to
The study catchments, like all tributary catchments in
Lodore and Whirlpool Canyons, are very steep, with
mean catchment slopes between 0.4 and 1.0 m m− 1
(Table 1). Tributary catchments have a large proportion
of their area at high elevations above the canyon rim
(Table 1, Fig. 4). Below the canyon rim, slopes and
tributary channels drop steeply to the Green River (Fig.
4). Debris flow initiation sites are located within the
steep portion of the watershed below the canyon rim,
where local slopes at initiation sites range from 0.5 to
1.2 m m− 1. There is not an inverse relationship between
slope and drainage area at the failure-initiation points in
this landscape (Fig. 5). This differs from landscapes
with soil or regolith-mantled hillslopes, where the local
slope at channel heads or debris-flow initiation sites
decreases with increasing contributing area (e.g.,
Montgomery and Dietrich, 1988; Cannon et al.,
2001b). In fact, slope remains fairly constant with
increasing contributing area, suggesting that slope alone
(and not contributing area) is a key physiographic
threshold variable for debris-flow initiation (Fig. 5).
Rock mass strength measurements show that the
cliff-forming bedrock units, regardless of being limestone or sandstone, are uniformly strong throughout the
study area. Mean Schmidt Hammer R values for the
cliff-forming bedrock units ranges from 50 to 63, and
they have similar weathering and jointing characteristics
I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
121
Fig. 4. (A) Catchment at river kilometer 381.8 in Lodore Canyon showing the locations of runoff generation, colluvium storage, and debris-flow
initiation that are representative of catchments underlain by the Uinta Mountain Group. Approximately 92% of the watershed burned in 2001.
Overland flow generated on bedrock slopes of the high elevation, upper catchment cascaded down a cliff onto colluvium stored on a bedrock bench.
The colluvium failed, initiating a debris flow that increased in volume as it scoured colluvium from down-slope deposits. Approximately 875 m3 of
material was deposited in the Green River, forming a new rapid. (B) Catchment at river kilometer 352.3 in Whirlpool Canyon showing typical
characteristics of catchments underlain by Paleozoic bedrock. The catchment was not burned. The debris flow initiated when overland flow generated
on shallow regolith and bedrock slopes poured down cliffs formed by the Weber sandstone and Upper Morgan Formations. Field observations
indicate that the overland flow poured over a 10-m bedrock step onto colluvium stored in a bedrock hollow at the base of the Upper Morgan
Formation, causing failure. The debris flow increased in volume as it scoured bed and bank material from a gully incised in colluvium deposited on
the lower-angle shale slopes of the Lower Morgan Formation before reaching the debris fan and Green River.
and are categorized as “strong” in the Selby (1980)
classification system (Table 2). Schmidt Hammer
measurements could not be obtained for slope-forming
shales, but ‘R’ values for incompetent rocks generally
range from 10 to 40 (Selby, 1980). The low strength and
the fissility of the shales place them in the “weak”
category (Table 2). The predominance of hard bedrock
in the study catchments clearly has an influence on
topography as well as sediment generation and
transport.
The presence of specific clay minerals has been
linked to debris flow activity in Grand Canyon. There,
abundant kaolinite and illite derived from terrestrial
shales is considered to be an important contributor to
debris flow mobility and long travel distances (Griffiths
et al., 1996). In our study sites, the clay-sized fraction of
colluvium and recent debris-flow deposits ranges from
7% to 23%. Together, illite and kaolinite comprised
nearly 100% of the three clay minerals we analyzed;
only two samples contained trace amounts of smectite
(Larsen, 2003). Surficial deposits derived from weathering of the Uinta Mountain Group contain predominantly kaolinite, whereas illite is more abundant in
surficial deposits derived from Paleozoic rocks.
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I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
Fig. 5. Slope versus contributing area for initiation sites of study debris flows. Initiation site slope changes little with increasing contributing area,
contrary to the inverse slope–area relation observed in regolith-mantled landscapes. This implies that slope specifically is the stronger physiographic
factor for debris-flow initiation.
5.4. Precipitation
Debris flows were triggered when daily rainfall
between 0 and 23 mm was measured at the nearest
weather station. These common rainfall events are all
smaller than the 28 mm, 2-year recurrence storm.
Hourly rainfall at the nearest RAWS stations ranged
between 1 and 11 mm, which was also less than the
15 mm, 2-h recurrence event. However, study catchments are no closer than 5 km to the weather stations,
and rainfall associated with the small-scale convective
storms that the triggered recent debris flows was
probably poorly represented by these distant gages.
Analysis of summer precipitation indicates that the
frequency of debris flows is not related to seasonal
precipitation (Fig. 6). However, record high precipitation occurred in some months when debris flows
occurred. Record high precipitation was recorded at
the Dinosaur Quarry area and Dinosaur National
Monument Headquarters weather stations in September
1997, while near-record rainfall occurred at the Brown's
Park Refuge weather station (Fig. 6). Rainfall associated
Table 2
Schmidt Hammer R values, joint characteristics, weathering characteristics, and calculated rock-mass strength for bedrock in Lodore and Whirlpool
Canyonsa
Parameter
Uinta Mountain Lodore Fm.
Group
Madison
limestone
Lower Morgan Fm. Upper Morgan Fm.
Lithology
Mean Schmidt
Hammer R
Weathering
Mean joint
spacing
Joint orientation
Width of joints
Continuity of
joints
Outflow of
groundwater
Total rating
Orthoquartzite
62 = 20
Sandstone
50 = 18
Limestone
63 = 20
Shaleb
5–10
Sandstone, limestone Sandstone
59 = 18
52 = 18
Slight = 9
27 cm = 17
Slight–moderate = 8
17 cm = 15
Slight = 9
26 cm = 17
Moderate = 7
0.2 cm = 2
Slight = 9
15 cm = 14
Slight–moderate = 8
15 cm = 14
Fair = 14
1–5 mm = 5
Continuous–
no infill = 5
None = 6
Favorable to Fair = 16
0.1–1 mm = 6
Continuous–
no infill = 5
None = 6
Fair = 14
Fair = 14
0.1–1 mm = 6
0.1–1 mm = 6
Few continuous = 6 Continuous–
no infill = 5
None = 6
None = 6
Fair = 14
0.1–1 mm = 6
Continuous–
no infill = 5
None = 6
Fair = 14
1–5 mm = 5
Few continuous = 6
76 = strong
74 = strong
78 = strong
72 = strong
71 = strong
a
b
Ranking from Selby (1980).
Shale strength values from Selby (1980).
45–50 = weak
Weber
sandstone
None = 6
I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
123
Fig. 6. Annual summer rainfall and the number of debris flows from 1993 to 2002 for Lodore and Whirlpool Canyons. Mean June–September
precipitation for the three weather stations is 60–110 mm. Record monthly precipitation occurred at the Dinosaur Quarry Area station in August and
September 1997 and July 2001. Record and near-record monthly precipitation was measured for September 1997 at the Dinosaur National Monument
Headquarters and Brown's Park Refuge stations, respectively. Wildfires occurred in 1996 and 2001.
with this September storm triggered three debris flows
(Table 1). However, other years with debris flows had
near-to-below average summer precipitation (Fig. 6).
5.5. Hillslope response to wildfire
The six study catchments that burned in 1996 and
2001 did so at high and moderate severities. Four of the
catchments burned at high severity, resulting in nearly
complete tree mortality and consumption of organic
litter. Two study catchments burned at moderate
severity, with only ∼50% tree mortality. These two
catchments are located in Lodore Canyon at river
kilometer 368.6 and 367.0 where the debris flows
initiated from shallow landsliding.
Thirty-nine of the 171 significant tributaries in
Lodore and Whirlpool Canyons were partially or
entirely burned and six (∼15%) of those produced
debris flows. Nine debris flows (∼7%) occurred in the
other 132 unburned catchments. The difference in the
frequency of debris flows from burned and unburned
catchments was not statistically significant (chisquare = 2.76, p = 0.097). However, when more than
half of the catchment area burned, a debris flow
generally occurred (Fig. 2). In regolith-mantled areas
that burned, the absence of rill and gully systems on
hillslopes suggests post-fire sediment bulking was not
the mechanism that generated the debris flows. Debris
flows in burned and unburned catchments initiated by
the firehose effect.
The higher proportion of debris flows from burned
catchments cannot be attributed to increased runoff
caused by soil water repellency. Soil water repellency
was not detected in the Yampa River tributaries that
burned at high severity during 2002 (Larsen, 2003).
Additionally, no debris flows reached the Yampa River
following the 2002 wildfire.
6. Discussion
6.1. Geologic controls on hillslope processes
Debris-flows in the eastern Uinta Mountains occur
primarily from factors associated with the geology and
topography of the landscape, whereas wildfires play a
secondary role. Hillslopes in tributary catchments have a
high proportion of exposed bedrock and lack continuous
colluvium (Figs. 3 and 4). The exposed bedrock leads to
high runoff ratios and streamflow generation in
ephemeral gullies that then pours over cliffs onto
colluvium, leading to failure and debris flows.
The steep topography of tributary catchments is a
function of the strong bedrock that is present throughout
the canyons. The topography, in turn, is an important
control on sediment transport processes. The slopes of
debris-flow initiation sites are all > 0.5 m m− 1, but
124
I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
contributing areas vary considerably, as is evident in the
lack of an inverse slope–area relation for debris flow
initiation (Fig. 5). An inverse slope–area relation does
not exist because debris flows initiate in the steepest
portion of the tributaries, between the canyon rim and
valley floor. The contributing area above the canyon rim
varies considerably among tributaries, and likely needs
to only exceed a threshold value in order to produce
sufficient runoff to trigger a debris-flow. Among the
study catchments, there is no threshold of drainage area
size where the dominant sediment transport process
changes from debris flow to stream flow, as has been
observed in the Yellowstone region (Meyer and Wells,
1997).
The canyons of the Colorado Plateau have similar
geologic, topographic, and climatic characteristics, and
debris-flow initiation via the firehose effect may be a
ubiquitous process throughout the Plateau. Debris-flow
processes in the eastern Uinta Mountains are similar to
those in Grand Canyon, where the firehose effect is also
the dominant debris flow initiation mechanism (Griffiths
et al., 2004). Dinosaur National Monument has strong
bedrock that forms cliffs and weak shales that form
slopes where colluvium is stored, similar to Grand
Canyon. Nearly all the recent debris flows in Dinosaur
National Monument initiated from colluvium failure,
similar to Grand Canyon where 78% of debris flows
initiate from colluvium (Griffiths et al., 2004). The
rainfall conditions in the eastern Uinta Mountains also
mimic those from Grand Canyon, where debris flow
occurrence is not related to seasonal precipitation and
most debris flows are triggered by localized summer
thunderstorms that effect only a few catchments (Webb
et al., 2000).
Debris flows that reach the Colorado River in Grand
Canyon most frequently initiate from distinct geologic
formations characterized by shales (Griffiths et al.,
2004). In contrast, debris flows that reach the Green
River in Lodore Canyon commonly initiate in catchments with little or no shale, though gradual slopes
underlain by shales are locally important for colluvium
storage. The clay mineralogy of colluvium in the eastern
Uinta Mountain is similar to that in Grand Canyon,
where illite and kaolinite are also abundant. However,
unlike Grand Canyon, clay-rich bedrock does not appear
to control the spatial distribution of debris flows
(Griffiths et al., 1996, 2004). Debris flows in the eastern
Uinta Mountains may reach the Green River despite the
contribution of abundant clay-rich material from shales
because of the relatively short travel distance between
initiation sites and the Green River. All of the recent
debris flows traveled less than 1.5 km, whereas debris
flows in Grand Canyon can travel as far as 22 km (Webb
et al., 1989).
6.2. Wildfire controls on hillslope processes
Wildfires are probably not the primary driver of
debris-flows in the eastern Uinta Mountains because
there was not a significant difference between the
proportion of debris flows from the burned and unburned
catchments that we studied. One of the main factors that
influence post-fire geomorphic response in a given
landscape is the degree to which physical processes are
regulated by vegetation (Swanson, 1981). If vegetation is
important, then fire has the potential to disrupt that role.
The dry climate causes hillslopes in the Uinta Mountains
to be sparsely vegetated. Burning has little effect because
vegetation and the organic litter it provides have little
influence on hydrologic and geomorphic processes.
Thus, hillslope response to wildfire in the eastern Uinta
Mountains is not as dramatic as in forests with regolithmantled hillslopes where burning can greatly increase
overland flow, erosion, and debris flow activity (i.e.,
Meyer and Wells, 1997; Cannon et al., 2001b). However,
debris flows were common when a high proportion of the
watershed area burned, suggesting that fire may play a
secondary role to geologic factors as a control on debris
flows. Removal of vegetation from hillslopes by burning
may increase overland flow by reducing interception and
causing soil sealing from raindrop impact. Potential
increases in overland flow from fires are likely small,
because the pre-fire vegetation density is already low and
hillslopes are primarily low-infiltration bedrock and thin
colluvium. However, small increases in overland flow
may lower the rainfall threshold for firehose-effect
triggered debris flows. The lack of rain gages in the
immediate study area makes it impossible to determine if
rainfall thresholds for initiation were lower in burned
catchments.
Few studies have addressed the impact of fire on
surficial processes in pinyon-juniper woodlands, but
results are generally consistent with our findings. Both
increased and decreased infiltration rates were measured
following a prescribed fire in a Nevada pinyon-juniper
woodland (Roundy et al., 1978). Little hillslope erosion
was measured following a wildfire in a pinion-juniper
woodland in Nevada, because pre-fire vegetation
densities were already very low (Germanoski and
Miller, 1995). However, significant entrenchment of
valley bottoms occurred following the fire because
removal of dense grass from valley bottoms lowered the
threshold for gully initiation (Germanoski and Miller,
1995). Hydrologic studies conducted in an unburned
I.J. Larsen et al. / Geomorphology 81 (2006) 114–127
New Mexico pinion-juniper woodland showed that
runoff from bare soil was 30–70% greater than from
vegetated patches (Reid et al., 1999). Therefore,
removal of vegetation by fire could increase runoff in
pinyon-juniper woodlands.
Though our observations are limited to 15 debris
flow events, additional evidence supports our finding
that fire has a limited influence on debris flow activity.
Previous accounts of debris flow activity in the eastern
Uinta Mountains have not cited fire as a contributing
factor (Graf, 1979; Hammack and Wohl, 1996) and the
period between 1997 and 2002 would have been an
increased period of debris-flow activity even without the
fire-related events (Fig. 6). The lack of debris flows
following the 19-km2 fire in catchments tributary to the
Yampa River also indicates that the link between debris
flows and fire is not as strong on the weathering-limited
hillslopes of the eastern Uinta Mountains as it is in
forested settings with transport-limited hillslopes.
7. Conclusions
We conclude that bedrock and topographic characteristics of the eastern Uinta Mountains are primary
controls on debris-flow initiation and that fire is a
secondary factor. The role of vegetation and fire in
regulating hillslope processes is similar to that of Grand
Canyon, where debris-flow activity is governed exclusively by geologic and physiographic factors (Griffiths
et al., 2004), but differs from that of forested ecosystems
with regolith-mantled hillslopes where fire can be the
dominant control on debris flow initiation (e.g., Meyer
and Wells, 1997; Cannon et al., 2001b).
Debris flows in the eastern Uinta Mountains initiate
from failure of colluvial wedges by overland flow via
the firehose effect and to a lesser degree, landsliding and
sediment bulking within channels. The most common
process linkage involves a sequence where i) rockfall,
rock avalanche, and mass-wasting processes transport
weathered bedrock from cliffs to colluvial deposits
located on bedrock benches, hollows, and shale slopes;
ii) summer storms produce infiltration-excess overland
flow on bedrock slopes in the upper elevation portions
of catchments; iii) overland flow cascades down the
cliffs, impacting and saturating colluvium; iv) colluvium
fails, initiating debris flows that increase in volume as
they scour material stored in downslope portions of the
catchment; and v) debris flows are deposited on fans or
in the Green River.
These processes are not primarily influenced by
wildfire because the dry climate results in hillslopes
with low vegetation density. Since vegetation does little
125
to regulate runoff and erosion, burning has a relatively
small effect on the hillslope processes that contribute to
debris flows. The main controls on debris flow activity
are bedrock and topographic properties. The combination of steep bedrock hillslopes, high runoff ratios, and
steep escarpments results in the initiation of debris flows
via the firehose effect. Debris flow initiation and
hillslope processes in Dinosaur National Monument
are similar to those in Grand Canyon, where bedrock
plays a similar role in forming steep slopes and
controlling colluvial sediment storage, but differs from
forested ecosystems where removal of vegetation by fire
is a primary control on debris flow initiation.
Acknowledgements
Funding was provided by the Colorado Scientific
Society and the Rocky Mountain section of the Society
for Sedimentary Geology (SEPM). We thank P. Kolesar
for assistance with clay mineral analysis and T. Larsen,
G. Larson, R. Heermance, and P. Petersen for field
assistance. T. Naumann of the National Park Service
provided invaluable logistical support. Reviews by R.
Webb, S. Cannon, and an anonymous reviewer greatly
improved this manuscript.
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